1. Field of the Invention
This invention relates generally to antenna systems for radiating and receiving RF energy and more particularly to an axial parasitically driven dipole antenna array.
2. Description of Related Art
As is well known, an antenna is an electrical element which can either radiate or collect electromagnetic energy. A transmitting antenna converts electrical energy from a signal source into electromagnetic waves of radio frequency (RF) energy which radiate away from the antenna either omnidirectionally or directionally depending upon the design. A receiving antenna, on the other hand, converts received RF energy into electrical energy which is coupled to RF receiver apparatus. Some antennas are adapted to serve both as transmitting and receiving antennas and are coupled to electrical apparatus which is adapted to both send and receive RF signals.
One such antenna comprises a half wave dipole antenna which consists of two quarter wave conductors linearly aligned and having the inner extremities which are excited by an RF generator. Such apparatus is well known to those skilled in the art and is well documented in the literature. Additionally, dipole antenna systems including one or more axially aligned dipoles for operating in the UHF and/or VHF frequency bands are also well known. One such antenna system is disclosed in U.S. Pat. No. 3,899,787, issued to W. P. Czerwinski on Aug. 12, 1975. The Czerwinski patent discloses a triplex antenna system comprising at least three individually excited tubular dipole antennas vertically oriented in an in-line configuration inside of a tubular radome and spaced approximately one wavelength apart. A coaxial sleeve approximately a quarter wavelength long is additionally mounted exteriorally of and is associated with each tubular radiating element inside of the radome for broadbanding the feed-point impedance of the respective dipole antennas.
Another example of an axial dipole antenna array is disclosed in U.S. Pat. No. 4,369,449, issued to J. B. McDougall on Jan. 18, 1983. There a linearly polarized omnidirectional antenna system is disclosed which includes one or more dipoles having an elongated tubular conductive radiator having a length that is about one half wavelength of the midband frequency and an elongated inner conductor member extending longitudinally through the interior of the radiator and spaced therefrom. A coaxial cable or other feed means conduct signals to and from one end of the radiator and to and from the inner conductor member. The impedances of the dipole and feed means are matched over a selected frequency band, such as by the use of a series inductive reactance between the feed means and the radiator. Two such dipoles can be connected to a colinear, center-fed pair, and two or more such dipoles can be arranged in the co-linear array having a common inner conductor member.
It is an object of the present invention, therefore, to provide an improvement in steerable axial dipole antenna arrays including active T/R modules which are powered by a DC power line consisting of a pair of elongated wire type conductors that tend to interact with the RF radiator so as to effectively short out the elements by the strong RF image produced by the electrically close DC wires.
Accordingly, this invention is directed to a method and apparatus by which the DC wires are used as part of the radiating system while maintaining DC continuity so that instead of shorting out the radiating elements, array performance is enhanced over the classic dipole array.
In one aspect of the invention, it is directed to an axial dipole antenna array, comprising: a plurality of spaced apart parasitically driven dipole antenna sections arranged linearly along a common longitudinal axis wherein each of the antenna sections include a pair of end loaded electrically short antenna dipole leg elements having an electrical length substantially less than a quarter wavelength (xcex/4), for example, less than one tenth wavelength (0.1xcex), a respective active transmit/receive module connected to the dipole leg elements and located in the immediate vicinity thereof, and a pair of capacitively coupled continuous electrical conductor members extending in an axial direction adjacent the dipole leg elements of the plurality of antenna sections for supplying DC power thereto and including RF chokes located adjacent the outer end portions of both of the dipole leg elements for restricting the electric length of the portion of the electrical conductors extending past the leg elements so that it is equal to or less than a half wavelength (xcex/2) for reducing the mutual coupling between the electrical conductors and the leg elements while at the same time forming a parasitic element for the respective dipole antenna section.
In another aspect of the invention, it is directed to a method of forming a dipole antenna array and comprises the steps of: arranging a plurality of parasitically driven dipole antenna sections linearly along a common longitudinal axis and where each of the sections include a pair of electrically short antenna dipole leg elements having an electrical length equal to or less than one tenth wavelength (0.1xcex), and loading the dipole leg elements with coiled inductive type elements, locating respective active transmit/receive modules in the immediate vicinity of the dipole leg elements, connecting the transmit/receive modules to the respective leg elements, installing a pair of capacitively coupled continuous electrical conductors in the axial direction adjacent the dipole leg elements for supplying DC power to the respective transmit/receive modules, and locating an RF choke immediately adjacent the outer ends of both dipole leg elements for restricting the electrical length of the portion of the electrical conductors extending past the leg elements so that it is equal to or less than a half wavelength for reducing the mutual coupling between the electrical conductors and the leg elements while forming a parasitic element for the respective dipole antenna section.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific examples, while disclosing the preferred embodiments of the invention, it is given by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art.